1. Field of Invention
The present invention relates generally to a receiver for receiving a modulated signal. In particular, the present invention relates to a digitally-implemented demodulator for demodulating amplitude-independent modulated signals. While the present invention has utility in any system involving the transmission and reception of signals characterized by amplitude-independent coding/modulation, the invention has been disclosed in both analog FM and digital FSK wireless receiver embodiments.
2. Background Art
Frequency and phase modulated signals have long been popular for use in communications systems due to their superior noise resistance and tolerance to varying and unpredictable channel propagation characteristics, as compared with amplitude-modulated signals. Frequency modulation continues to be a very popular modulation scheme for the transmission of voice and music is systems employed by commercial radio stations, cellular telephones, and cordless telephones. Similarly, the easily-implemented Frequency Shift Key (“FSK”) modulation schemes have become popular for the transmission of digital data in numerous contexts, such as telephone line modems, cellular telephones, and cordless telephones. Finally, phase modulation schemes such as BPSK and QPSK are integral to many very sophisticated modern communications systems, such as the CDMA digital cellular telephone standard.
While such modulation techniques have desirable characteristics, many conventional receiver and demodulator circuits have relied in large part on analog circuit components. While consumers demand wireless products having increasingly compact form factors, analog circuits incorporated in such products typically are often large, with high numbers of discrete parts. Furthermore, analog components tend to be subject to significant variations in manufacturing tolerances and are sensitive to environmental conditions and aging, which may degrade performance. A digitally-implemented receiver, however, would not be subject to such variability. Furthermore, a digital receiver could be integrated into a single application specific integrated circuit (ASIC), allowing a potentially drastic reduction in overall receiver size and part count. Therefore, it is an object of the present invention to implement a demodulator digitally.
Furthermore, prior art demodulators commonly require tuning or calibration during the manufacturing process, adding to the time and cost required to produce the products into which they are incorporated. It is a corresponding object of the invention to minimize or eliminate the tuning or calibration required during manufacture of a demodulator according to the present invention.
Receivers of conventional design typically utilize an automatic gain control amplifier to maintain the received signal within the dynamic range of the RF circuit. However, such receivers are typically limited in their reaction time, and may not always adjust quickly enough to accommodate drastic signal level transitions, such as may be caused by a frequency hopping receiver that hops into a null. Accordingly, it is an object of the present invention to implement a receiver that does not require an automatic gain control amplifier.
These and other objects of the present invention will become apparent in light of the present specification and drawings.